The shortage of food in the world recently
prompted the Director General of FAO, Edouard Saouma, to
reiterate the special need for food in densely populated rural
areas of developing countries (1). We need new food sources, and
we should not restrict ourselves to increasing supplies of
existing ones to meet this demand (2). In our attempts to develop
these potentials we should, however, avoid theoretical overkills
(3).

In this paper, I shall try to take these points
into account while studying the question of whether new sources
can be tapped to a significant extent, and whether these new
rural sources can provide food that is affordable, whole some,
and acceptable organoleptically. In view of the latter point, I
would like to emphasize that, especially in rural areas,
consumers are extremely critical. This is by no means limited to
developing countries only. In the Netherlands, too, children are
taught, "What a farmer is not familiar with, he does not
eat."

An inventory of nutrient sources is rather
illuminating (see Figure 1). There are approximately 9,000
million ha of land in the world. Close to 50 per cent of this
area consists of forests and shrub lands. Another 35 per cent is
pasture and grassland, and 15 per cent is arable land. Of the
produce grown on arable land, by far the major part is discarded
as residue. This means that approximately 95 per cent of the land
areas mentioned in Figure 1 could provide a source of nutrients
yet untapped in the sense that their agricultural residues are
not being utilized directly for human consumption.

Food science and technology at present
concentrate mainly on the 5 per cent of the annual production of
potential nutrients that can be used after relatively simple
forms of processing, such as cooking or baking. The 95 per cent
residue needs considerable processing, be it physical, chemical,
or via some form of bioconversion before it can be turned into
suitable feed or food. This paper will concentrate on the
products that we refer to as organic residues. They are what is
left after agricultural production, sometimes left behind on the
land (straw), on the farm (manure), or in agro-industries. They
are relatively easily available for conversion into food.

Figure 2 shows some of the most important
agricultural crops. There is a line along which we can divide
these products. On the left side we find the percentage
considered to be the main food component of the crops. They can
be used without much processing by the human consumer (grain,
oil, starch, vegetable protein). On the right side are the
residues that comprise approximately two-thirds of the total
production.

Two routes are available to convert these
residues into useful products, as indicated in Figure 2: through
feeding to animals, or by some industrial process.

Because these residues form the major part of
agricultural production, their conversion into food via efficient
and safe systems deserves far more attention than we have paid it
so far.

Tremendous efforts are required to make such
new systems operational. Therefore, we should concentrate on a
limited number of the most promising ones and approach them in a
multi-disciplinary fashion.

Those of us involved in developing new
conversion systems will agree that the creation of acceptable
food from novel sources usually requires animal conversion as a
last step. Microbial conversion alone produces, in most cases,
biomass, e.g., single-cell protein that is not accepted as a food
by most consumers.

To identify the most suitable areas in which
bioconversion of residues may be important and therefore
worthwhile, the residues are divided into three categories, each
susceptible to a common method of bioconversion. Cellulose-rich
substrates form a total of more than 1,800 million tons annually
of renewable resources (Table 1). They are to a great extent
found in Asia, and it is therefore not surprising that they
consist primarily of rice straw. The present use is often none;
in some areas it is used for fuel. Straw could form an extensive
base for reeding ruminants. There is no doubt that bioconversion
would greatly improve the use of these materials, particularly in
rural areas. To what extent in vitro SCP (single-cell protein)
production can play a major role here depends greatly on local
circumstances and on the results of research efforts in this
field.

TABLE 1. Straw Production, 1974 (in millions of
tons)

Crop

World

Africa

South America

Asia

Paddy rice

323

8

10

294

Wheat

360

8

10

90

Maize

586

54

58

100

Other straws

441

41

18

123

Total straws

1,710

111

96

607

Sugar-cane

116

9

28

46

Total

1,826

120

124

653

The second major residue category consists of
starchy and sugary wastes (Table 2). Because their carbohydrates
are more easily accessible, they require a somewhat less
difficult form of SCP bioconversion. As shown in Table 2, cassava
and sugar beets provide the greatest amount of residue. High
productivity in relatively poor soil has made cassava a popular
staple food, especially in countries most in need of food.

TABLE 2. Starchy, Sugary Residues (in millions
of tons)

Crop

World

Africa

Latin America

Asia

Cassava

106

42

33

30

Sugar beets

482

4

5

39

Bananas

8

1

4

2

Citrus fruits

12

1

3

3

Coffee

5

2

3

-

Total

613

50

48

74

A third category of residue is manure, a
by-product of all animal production systems. It is calculated
that approximately 1,900 million tons of manure are produced per
year.

Residues are not used as foods because they are
inedible without some form of bioconversion. Table 3 shows that
the chemical composition of most residues is not well balanced.
Straw contains 48 per cent crude fibre and 3 per cent crude
protein.

It is hardly a good product for human
consumption. Grass has a better composition, but if it were to be
used for monogastric consumers like man, there would still be
severe problems because of the relatively high crude fibre
content. Poor digestibility is another reason for rejecting
unprocessed residues.

Table 4 gives the digestibility coefficients of
some residues for ruminants. The organic matter of straw is only
38 per cent digestible even for ruminants. For monogastric
organisms like man, poultry, and pigs, the coefficients are even
lower. Grass is more digestible but less suitable for
monogastrics. Digestibility of citrus and animal wastes is
reasonable, but not particularly good.

A number of other reasons may make a residue
undesirable. Logistic aspects and low drymatter content may be
expensive to overcome. Seasonal variability often makes it
difficult to manage the material by advanced technology. Chemical
and microbial contamination and organoleptic or psychological
unacceptability may preclude the use of some residues as food.
The above characteristics all present problems that must be
overcome if a residue is to be converted to food.

Figure 3 shows the pathways for the
bioconversion of residues into food. In the upper box,
cellulose-rich, starchy and sugary residues, and animal manure
are represented. The lower box shows the goal of bioconversion
systems: food for man. In most cases this will be in the form of
meat, milk, or eggs.

It is frequently said that there seems to be a
certain competition for food between animals and man. One easily
overlooks that this is the exception rather than the rule. Most
animals are kept for the purpose of producing food for man. They
are mainly converters (biological ones) of products inedible by
man. As such, they do not compete significantly for human food
supplies.

We have many options for making food from
wastes. The ones by-passing the animals are represented by the
dotted lines in Figure 3. Direct use as food is non-existent,
otherwise the product would not be a waste. Chemical and physical
treatments of waste seldom create food. Microbial conversion,
either direct or after treatment, permits mushrooms to grow and
favours the production of fermented oilseed cakes. Unfortunately,
this method is not yet used for the conversion of millions of
tons of residues to any significant degree.

The solid lines on the right side of the figure
represent bioconversion systems making use of animals. Grass,
straw, and quite an amount of poor-quality roughage follow the
direct route to food. Animal feeds may also be wastes that are
treated via chemical or physical means and/or by microbial
conversion, which the animal also converts to food. The potential
and efficiency of bioconversion should be exploited to a much
greater degree. In general, the right side of Figure 3 shows the
most realistic potential for bioconversion of the bulk of
residues into food.

The alkali treatment of cellulose-rich
materials like straw (1,800 million tons in rural areas) deserves
special attention (Table 5). Digestibility for ruminants improves
from 45 to 68 per cent when straw is treated. What does that
imply? in the major rural areas of India, untreated rice straw
provides hardly enough nutrients to maintain the live weight of
cattle; in other words, it barely covers the animals' maintenance
requirements Assuming that 90 per cent of the feed is used for
maintenance, 10 per cent is available for increasing weight, for
producing offspring, and for milk production. If digestibility
were increased by 50 per cent, it would provide, as in the Table
5 example, a fivefold increase in nutrients available for meat
and milk production.